. The Canadian field-naturalist. 410 The Canadian Field-Naturalist Vol. 88 When a ground squirrel was caught it was weighed to the nearest 5 grams on a spring scale, sexed, classified as adult or young of the year on the basis of size, and marked with a numbered ear tag (National Band and Tag Company) in each ear. Females with enlarged mammae or patches of dark or matted fur around the mammae were classified as lactating. After processing, the ground squirrel was released into the burrow from which it had come and the traps were removed. Results and Discussion Annual Activity Cycle Each trappi
. The Canadian field-naturalist. 410 The Canadian Field-Naturalist Vol. 88 When a ground squirrel was caught it was weighed to the nearest 5 grams on a spring scale, sexed, classified as adult or young of the year on the basis of size, and marked with a numbered ear tag (National Band and Tag Company) in each ear. Females with enlarged mammae or patches of dark or matted fur around the mammae were classified as lactating. After processing, the ground squirrel was released into the burrow from which it had come and the traps were removed. Results and Discussion Annual Activity Cycle Each trapping season was divided into weekly periods with week 1 beginning on 1 April. Figure 1 shows the proportions of the total weekly catch, including recaptures, by age and sex. In the first week of April adult males made up the majority of the population, suggesting that males emerge ear- lier from hibernation than females. The proportion of females increased rapidly as they emerged from hibernation. During the first half of July the pro- portion of adult males decreased to nearly zero. It is assumed that the males were entering hibernation, not dispersing, because individuals were recap- tured in the subsequent summer at the original locations. The adult female proportion approached zero by the end of July when females entered hibernation. In 1969 the first young ground squirrel was trapped on 26 May, in 1970 on 31 May, and in 1971 on 4 June. G. R. Michener (1973) observed young on 29 May 1971 on the study area. The data support G . R. Michener's (1973) estimate that breeding occurs in early April when females emerge from hibernation. The proportion of young being trapped increased rapidly for 2 weeks as litters reached the stage of above-ground activity. By late August young constituted 100% of the active population. The ratio of male to female young remained approximately 1:1 from emergence until the last half of August. In late August the proportion of young males increased rela
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